I finished Jonathan Strange and Mr. Norrell today, and I liked it very much. I recommend it for both people who aren't going to read the book, and for people who have already done so. (I'm undecided on whether it'd be better to put off watching the show until you've read the book, if you're ever going to do that, but if you're not worried about it, I'd say go for it.)
There were a lot of things missing from the book. Obviously. There would have to be, because the book is gigantic. The things they skipped or simplified were, in my opinion, very well chosen -- the quantity of stuff that happened each episode was very satisfying, and while I would definitely have been on board for a six season exploration of every page and footnote of the entire novel, I understand the need to make TV shows tolerable to people who want to see a plot moving forward, and they did an excellent job of adapting for that purpose.
Strange and Norrell are brilliantly cast and played, by the way. It was very exciting to see those characters portrayed on screen.